Ink jet printers are known in the art and include those made by Hewlett-Packard, Canon and Epson, among other producers. Ink jet printers function by several actuation mechanisms, including thermal (heating resistor) or mechanical (piezo-electric) actuators. While the discussion herein is primarily directed toward thermally actuated printheads, it should be recognized that the varied passivation layer thickness of the present invention are also applicable to mechanically actuated printheads. As discussed in more detail below, the present invention is concerned with providing a thick passivation layer to protect circuitry on a printhead die, while providing a thin passivation layer over the ink expulsion element to reduce ink expulsion energy. A thin passivation layer reduces the energy required to expel ink, regardless of the type of actuator and thus the present invention is applicable to all ink jet and related printers.
FIG. 1 illustrates a representative printhead structure of a prior art ink jet printhead that is thermally actuated. The structure of FIG. 1 includes a substrate 10 usually of semiconductive material in which is formed a resistive layer and element 12. A layer of conductive material 14 (usually aluminum or the like) is formed on the substrate, generally as shown. A passivation layer 20 (normally Si.sub.3 N.sub.4 /SiC or the like) is formed on the substrate, and a metallic layer 26 and contact pad 28 (coupled through via 25) are formed on the passivation layer. The metallic or conductive layer may include a protection/cavitation layer 24 and a surface conductor 26. An inkwell 31, barrier layer 32 and orifice plate 33 are provided as is known. A printhead "fire" signal is propagated from circuit 50 or from an off-chip source to the resistive element and there produces sufficient heat to cause a drop of ink to be expelled through the orifice plate 33.
The amount of energy required to expel a drop of ink is often referred to as the turn-on energy (TOE). TOE is related to passivation layer thickness in that the thicker the passivation layer, the more energy required to expel a drop of ink. Thus, to reduce TOE a thin passivation layer is desired.
A thin passivation layer, however, has disadvantageous aspects. One disadvantageous aspect is that as the passivation layer thickness is reduced, the likelihood of a passivation layer crack or other defect increases. To minimize the possibility of passivation layer cracking, steps such as beveling the transitions of the underlying topology, particularly those near the resistive element (which is a place of higher physical stress) have been undertaken. For example, edges 13,15 of the conductive layer 14 proximate resistive element 12 may be beveled. While beveling reduces physical stresses on the passivation layer, it is significantly more difficult to precisely position a beveled edge than to position a straight (vertical) edge. The significant margins of error in beveled edge placement result in significant variability in the defined resistor size and amount of heat generated thereby. This in turn results in inconsistent firing of the printhead and inconsistent print intensity, among other problems.
Another disadvantageous aspect of a thin passivation layer relates to the expanded use of the printhead die or substrate 10 for processing logic 50. As the number of individual firing chambers in a printhead die increases, the number of power conductors and signal conductors for these firing chambers increases. These conductors are usually formed on top of the passivation layer. As passivation layer thicknesses decrease and the provision of surface conductors increases, the likelihood of capacitive coupling or the like effecting circuitry within the substrate increases. Thus, in order to protect circuitry within the substrate, it is necessary to have a sufficiently thick passivation layer. As stated above, however, increasing passivation layer thickness disadvantageously increases the TOE.